Plated zinc base articles and method of making



Nov. 1, 1966 HARROVER, JR 3,282,659

PLATED ZINC BASE ARTICLES AND METHOD OF MAKING Filed Aug. 24, 1965 WITNESSES INVENTOR Rober'r E. H0rrover,Jr.

United States Patent Ofiice 3,282,659 Patented Nov. 1, 1966 3,282,659 PLATED ZINC BASE ARTICLES AND METHOD OF MAKING Robert E. Harrover, In, Baltimore, Md., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 24, 1965, Ser. No. 490,760 14 Claims. (Cl. 29-1835) This application is a continuation-in-part of my copending application Serial No. 284,622, entitled Plated Zinc Base Articles and filed May 31, 1963, and now abandoned.

This invention relates in general to plated zinc base articles and more particularly to improved composite plating deposits on zinc and zinc alloy castings that include layers of copper, nickel and chromium.

A great number of articles that are fabricated from zinc base metal compositions, especially zinc and Zinc alloy diecastings for example, are exposed to outdoor environments The articles are provided with surface finishes to inhibit, reduce or eliminate the corrosion associated with outdoor exposure. One of the most popular surface finishes for these articles is a composite coating of copper, nickel and chromium. For example, a standard procedure for finishing zinc alloy diecastings includes the steps of polishing or buffing, degreasing, cleaning and sequentially electrodepositing layers of copper, nickel and chromium.

A pronounced disadvantage attends the zinc base articles plated with the described composite coating when exposed to outdoor environments for prolonged periods. The typical corrosion failure includes unsightly blisters. Those skilled in the art recognize that after sufficiently prolonged exposure, failure of some kind must be accepted as inevitable for electrodeposited coatings. Accepting failure ofsome kind as inevitable, it would be more desirable if it were to take some form other thanblistering where a decorative or ornamental surface finish is a requisite.

Accordingly, it is a general object of this invention to provide zinc base articles with composite surface coatings or finishes having an attractive decorative finish.

Another object of this invention is to provide an attractive decorative surface for zinc base articles that will remain attractive even after prolonged exposure to outdoor atmospheres.

A further object of this invention is to provide a composite surface finish for zinc base diecastings that will not blister after prolonged outdoor service.

Yet another object of this invention is to improve the decorative service life of copper-nickel-chromium plated zinc and zinc alloy diecastings.

Briefly, the present invention accomplishes the above cited objects by interposing a layer of material that will act as a diffusion barrier between the surface of the zinc base article and the copper layer. The decorative surfaces of zinc base article must be completely covered by the diffusion barrier. By depositing the diffusion barrier between the zinc and copper, as for example a continuous layer of nickel or iron with layers of copper, nickel and chromium thereover, the blistering normally associated with the prolonged exposure of decorative zinc base articles is essentially eliminated.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawing, in which the figure is an enlarged cross-sectional view of an article with the decorative composite coating of the invention.

It has now been discovered that the blister type of corrosion failure normally associated with copper, nickel, chromium plated zinc base articles may be essentially eliminated by intcrposing a material which will reduce or eliminate the difiusion growth of a brittle copper-zinc alloy layer at the copper plate-zinc interface. Thin layers of iron and nickel have been found to be satisfactory means of stopping the growth of the brittle copper-zinc alloy layer which is formed by diffusion between the copper and zinc, even at room temperature. The thin layers of iron and nickel serve as means for improving the service life of plated zinc articles.

The corrosion mechanism for the initial deterioration of copper, nickel, chromium plated Zinc diecastings is attributable to the unfavorable nickel-chromium galvanic couple. The corrosion starts in the nickel plate at crack sites in the chromium overlay and eventually penetrates the nickel, thus exposing the copper plate. In time, especially in outdoor environments, the copper plate is also perforated. The diecasting itself is then exposed and the rapid corrosion of the zinc follows. The voluminous zinc corrosion products tend to seal the perforations and, when this occurs, pressure is exerted on the surrounding area which has been undercut by the rapid corrosion of the zinc diecasting. This pressure causes the electroplate to yield locally and an unsightly blister is raised on the surface of the plated diecasting.

It is believed that the failure of the electroplated coating under the described pressures indicates that it does not have sufficient mechanical strength. The formation of the brittle copper-zinc diifusion layer materially reduces the ability of the coating to resist this pressure and is believed to be the prime factor leading to the blister type of failure of copper, nickel, chromium plated zinc diecastings. Inhibiting the formation of the brittle alloy layer will. therefore, enhance the decorative service life of the plated zinc diecastings.

The invention may be further described and illustrated by the following specific examples, which should in no way be construed as limitations of the scope of the invention.

A typical zinc base diecasting alloy consisting, on a weight basis, of 3.5 to 4.3% aluminum, 0.03 to 0.08% magnesium, the remainder being zinc and normal impurities, was employed to fabricate a plurality of 2 inch x2 inch x inch test panels. In preparation for plating, the test panels were buffed, degreased by soaking in benzene, cleaned anodically, rinsed, acid dipped and rinsed again. All test panels employed hereinbelow were subjected to this identical treatment.

EXAMPLE I Test panels were plated in the Cyanide Copper Strike Bath of Table I hereinbelow, for one minute with a current density of 40 a.s.f., no agitation and with the bath at F. The panels were then plated in the P-R Copper Bath of Table I for 40 minutes with air agitation and the bath at 180 F. The current was periodically reversed, the panels being alternately cathodic for 60 seconds at a current density of 60' a.s.f. and anodic for 45 seconds at 45 a.s.f. The total thickness of copper on each panel was about 0.001 inch. The panels were then further plated in the Bright Nickel Bath of Table I at F, employing a current density of 5 0 a.s.f. with air agitation for 10 minutes. A total thickness of about 0.0002 inch of nickel was deposited over the copper plate.

The panels were heated in a mechanical convection furnace at 355 F. for various times from 1% hours to 64 hours to accelerate diffusion. Each panel was sectioned and examined metallographically for evidence of double chromium plating thicknesses, for example, are becoming more common specifications commercially and are non-embodied in ASTM specifications. Copper, nickel and chromium might, for example, run as high as 0.005, 0.005 and 0.0001 inch respectively. The nickel and chromium, particularly, may be deposited in multiple layers with the known attending advantages of multiple coatings.

While there has been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to .those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. An article fabricated from a metal selected from the group consisting of zinc and zinc alloys being plated with an electrodeposited protective decorative coating of respective layers of (1) a barrier metal selected from the group consisting of nickel and iron, (2) copper up to about 0.005 inch thick, (3) nickel up to about 0.005 inch thick and (4) chromium up to about 0.0001 inch thick.

2. The article of claim 1 in which the barrier metal is nickel.

3. The article of claim 1 in which the barrier metal is iron.

4. The article of claim 1 in which the barrier metal is from about 0.00004 to 0.00009 inch thick.

5. An article fabricated from a metal selected from the group consisting of zinc and zinc alloys being plated with an electrodeposited protective decorative coating of respective layers of (l) a barrier metal selected from the group consisting of nickel and iron, (2) copper up to about 0.001 inch thick, (3) nickle up to about 0.00025 inch thick and (4) chromium up to about 0.00005 inch thick.

6. The article of claim 5 in which the barrier metal is nickel.

7. The article of claim 5 in which the barrier metal is iron.

8. The article of claim 5 in which the barrier metal is from about 0.00004 to 0.00009 inch thick.

9. In a decoratively finished article fabricated of a base metal selected from the group consisting of zinc and zinc alloys and having surfaces upon which are deposited respective layers of copper, nickel and chromium, the improvement comprising interposing, between said base metal and said copper layer, a metal barrier layer selected from the metals consisting of nickel and iron, thereby preventing the development of unsightly corrosion products.

10. The article of claim 9 in which said metal barrier layer is nickel.

11. The article of claim 9 in which said metal barrier layer is iron.

12. In the method of decoratively finishing articles composed of a metal selected from the group consisting of zinc and zinc alloys to prevent the occurrence of unsightly corrosion products, the improvement comprising sequentially electrodepositing on the surface of the article a first or barrier layer of a metal selected from the group consisting of nickel and iron and subsequent layers of copper, nickel and chromium.

13. The method of claim 12 in which the first or barrier layer is nickel.

14. The method of claim 12 in which the first or barrier layer is iron.

References Cited by the Examiner UNITED STATES PATENTS 2,120,738 6/1938 Domrn 29196.5 X 2,128,550 8/1938 Ford. 2,658,266 11/1953 De Rose.

HYLAND BIZOT, Primary Examiner. 

1. AN ARTICLE FABRICATED FROM A METAL SELECTED FROM THE GROUP CONSISTING OF ZINC AND ZINC ALOYS BEING PLATED WITH AN ELECTRODEPOSITED PROTECTIVE DECORATIVE COATING OF RESPETIVE LAYERS OF (1) A BARRIER METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND IRON, (2) COPPER UP TO ABOUT 0.005 INCH THICK, (3) NICKEL UP TO ABOUT 0.005 INCH THICH AND (4) CHROMINUM UP TO ABOUT 0.0001 INCH THICK. 